The present invention concerns a process for making polyethylene naphthalate and polyethylene terephthalate blends, and more particularly to a method of controlling the change of intrinsic viscosity and level of transesterification during solid stating of such blends.
Polyethylene naphthalate (PEN) has a significantly higher glass transition temperature (Tg) than polyethylene terephthalate (PET), i.e., about 120xc2x0 C. compared to 80xc2x0 C., as well as a five time improvement in oxygen barrier property. PEN is thus a desirable polymer for use in thermal-resistant beverage containers (e.g., hot-fillable, refillable and/or pasteurizable containers), and for packaging oxygen-sensitive products (e.g., beer, juice). However, PEN is more expensive (both as a material and in processing costs) than PET and, therefore, the improvement in properties must be balanced against the increased expense.
One method of achieving an article that is lower in cost than PEN, but with higher thermal and barrier properties, is to provide a blend of PEN and PET. However, blending of these two polymers often results in an opaque material with incompatible phases. Efforts to produce a clear container or film from a PEN/PET blend have been ongoing for over ten years, but there is still no commercial process in widespread use for producing such articles.
One suggested method for making substantially transparent PEN/PET blends is a solid-stating process which increases the level of transesterification (copolymerization) between the two polymers. For example, WO 92/02584 (Eastman) states that transesterification occurs when the melt blended, crystallized polymer is held at a temperature below the melting point and subjected to an inert gas flow in order to raise the inherent viscosity and/or remove acetaldehyde. This transesterification is in addition to that occurring during melt blending and molding operations. However, Eastman reports that when the level of transesterification between the two polymers is very high, the crystallinity and resultant physical properties of the blend are reduced to the point where they are undesirable for making oriented containers with good mechanical properties.
Eastman teaches the addition of a phosphorus stabilizer for controlling (reducing) the amount of transesterification which occurs during solid stating. In this way, Eastman claims to limit the amount of transesterification to an amount no greater than about 20%, based on a theoretical maximum amount of transesterification being equal to 100%. For example, in Table 2 Eastman describes the transesterification and inherent viscosity of various solid-stated PEN/PET blends, where the initial inherent viscosity of the blend was on the order of 0.55 to 0.65, and the final inherent viscosity was about 0.80 to 0.85. In a control example (50xe2x80x9450 PEN/PET) the final inherent viscosity was acceptable (0.86) after eight hours, but the percent transesterification (25.0) was too high (above 20%). By adding 0.5 or 1.0% Ultranox 626 (a phosphite stabilizer) in the first two examples, the Eastman process provided a final inherent viscosity of 0.80 to 0.84 after eight hours, and an acceptable percent transesterification of 17.0 or 19.0 (below 20%). The other three stabilizers/metal deactivators tested in Table 2 failed to provide the final desired inherent viscosity and transesterification levels.
Although the Eastman process may be suitable for certain limited starting materials and desired transesterification levels, it cannot be expanded generally to different combinations of intrinsic viscosity, solid-stating time, and levels of transesterification. For example, of potential interest is a blend made from precursor homopolymer PEN and post-consumer PET (PC-PET). The intrinsic viscosity of PC-PET is much higher than that of virgin fiber-grade PET, so that a blend of PEN/PC-PET would require a relatively larger amount of transesterification per unit intrinsic viscosity increase (compared to a blend of PEN/virgin PET). Hence, among other disadvantages, the prior art does not provide a process that allows a desired level of both intrinsic viscosity and transesterification level.
It is possible to make substantially transparent preforms (for blow molding into containers) with a PET/PEN blend, without solid stating, but the disadvantages are such that the process is not commercially viable. First, the preform injection molding temperature (i.e., barrel temperature) and/or the equilibration time (i.e., time in the barrel) must be increased such that the resulting process is not cost-efficient or sufficiently reproducible for a commercial process. For example, in certain cases, the barrel time would be increased by a factor of four (i.e., an increase over the standard cycle of 45 seconds of up to 180 seconds); as a result, one would probably not be able to run the process on a standard injection molding machine. Furthermore, the increase in barrel time/temperature increases the acetaldehyde (AA) levels in the preform to an unacceptably high level, such that AA is likely to be extracted into the food product and produce an off taste, particularly with a product such as bottled water. Thus, this has not proven to be the desired solution.
According to the present invention, a process is provided for controlling both the rate of change of intrinsic viscosity (IV) and the rate of transesterification of a blend of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) during solid stating. The method comprises providing PEN having a first intrinsic viscosity (IV), providing PET having a second IV, and reacting the PEN and PET in the presence of an ethylene glycol compound in an amount sufficient to achieve a desired final IV and final level of transesterification in the copolymerized PEN/PET product.
In one embodiment, a full-length preform sleeve layer is made from a PEN/PET blend having an effective amount of ethylene glycol to increase the Tg at least about 15xc2x0 C. Other layers of the preform body may be PET. In this embodiment, a moderate, controlled level of transesterification is provided to enable strain orientation/crystallization in both the blend and PET layers for optimizing the mechanical performance, while maintaining optical clarity (substantial transparency).
In another embodiment, the process is used for making container preforms having a neck finish with a transesterification level of at least about 30% or greater. For example, a 30% PEN and 70% PET weight percent blend includes an effective amount of ethylene glycol to obtain a desired high level of transesterification, but without raising the molecular weight (i.e., intrinsic viscosity) too high. This blend will provide a high Tg neck finish portion and is also melt compatible with adjacent PET layers to maintain clarity and adhesion. Because the neck finish is not stretched, there is no need to provide a lower level of transesterification as would be required to enable strain orientation/crystallization.
In other embodiments, the method of this invention enables the use of initial higher molecular weight polymers. For example, it may be desirable to utilize post-consumer PET (PC-PET), having an initial IV of 0.72-0.73 dL/g, in an amount of from about 60-90 weight percent, with the remaining component being PEN. A predetermined final IV and transesterification level are achieved by adjusting the solid stating time and/or amount of ethylene glycol used.
The alkylene glycol preferably has up to 6 carbon atoms, more preferably 2 or 3 (propylene or ethylene), and more preferably 2 (ethylene). It may be precompounded with the PET and PEN, or added to the reaction chamber in which the PET and PEN are copolymerized. Preferred amounts of the alkylene glycol include at least 0.05 weight percent based on the total weight of PET and PEN, more preferably 0.1 to 2 weight percent, and most preferably 0.1 to 0.5 weight percent.